In the middle of London’s rush hour on Tuesday, a man crashed his car into the security barriers outside of the Houses of Parliament. Police are treating the incident, which left three people injured, as a potential terrorist attack.

President Trump’s response was quick and followed his usual pattern: He fired off an early-morning tweet about “another terrorist attack” in London, comparing the suspect to an animal.

Another terrorist attack in London...These animals are crazy and must be dealt with through toughness and strength!

A sense of deja vu is understandable. A little over a year ago, a similar attacker — using a vehicle and then a knife — killed five people and injured more than 50 others outside the Houses of Parliament and the adjacent Westminster Bridge. Trump also tweeted (twice) about that attack — among the 46 tweets mentioning “terror” he made in 2017.

Despite such high-profile attacks and responses, however, statistics released this month by the University of Maryland suggest that 2017 was the third consecutive year that the number of terrorist attacks around the world — and the deaths caused by them — had dropped. So far, 2018 looks on track to be lower still.

The university’s Study of Terrorism and Responses to Terrorism (START) program found there were 10,900 terrorist attacks around the world last year, which killed a total of 26,400 people, including perpetrators. That was a drop from 2016, which was in turn a drop from 2015. Right now, the number of terrorist attacks and deaths from terrorism appears to have peaked in 2014, when there were nearly 17,000 attacks and more than 45,000 victims.

What explains the downward trend? In Western Europe, where the number of attacks increased slightly in 2017 — but the number of victims dropped by 65 percent — it may be matters of policing and counterintelligence. British Prime Minister Theresa May said Tuesday that the country had foiled 13 Islamist terrorism plots and four far-right plots since March 2017, when last year’s attack near the Houses of Parliament occurred.

On a global scale, however, the answer is clear. Though there was a surge in terrorist attacks in Europe in recent years, most attacks still occur in the Middle East and Africa, and those regions saw a big decline in 2017. The number of terrorist attacks in the Middle East and North Africa dropped by 38 percent year on year according to START; the number of victims declined by 44 percent.

That can largely be attributed to the Islamic States' loss of territory and military defeats throughout 2017. Without a stable base, the number of attacks the jihadist group could stage in countries like Iraq and Syria dropped sharply, as did the damage it could inflict on civilian populations.

Indeed, it was largely thanks to the rise of the Islamic State and other extremist groups like Boko Haram that 2014 became such a remarkable year for global terrorism in the first place. In 2015, the Institute for Economics and Peace released a report that found there was an 80 percent increase in the number of deaths from terrorism in just one year — and the number of deaths from terrorism in 2014 was ninefold what it had been in 2000.

But a closer read of the 2014 statistics paints a nuanced picture. START’s data from the year found just three countries — Iraq, Nigeria and Afghanistan — accounted for 60 percent of deaths from terrorism. Even though the total number of deaths globally from terrorism that year was clearly terrible (more than 45,000, according to START), it was still less than the number of deaths due to drug overdoses that same year in the United States alone.

The United States was largely spared from the worst of this global increase in terrorism — START found 27 deaths in the country from terrorism in 2014, which included perpetrators. But terrorism has still shaken American politics. A Pew poll from July 2016, for example, found that voters thought that terrorism should get more time than any other subject in presidential debates.

It’s too early to say whether 2018 will continue the encouraging trend of the past three years. Another monitoring group, Jane’s IHS Markit, has warned that there may be a surge in attacks in Europe as Islamic State fighters return from the Middle East. Notably, on Tuesday, two separate reports suggested the number of fighters still with the Islamic State in Iraq and Syria is considerably higher than previously estimated.

But a bigger question is whether we can even know that terrorism is declining. Measuring global terrorism is a complicated task: START is one of only a handful of projects that strives to monitor it on a global level (Jane’s, mentioned above, is another).

For the past six years, the State Department provided funding to START, effectively subcontracting the job of data collection on terrorism. But Erin Miller, the program manager for START’s Global Terrorism Database, said in an email that her team has been told the State Department did not renew its contract. Though the team is now seeking alternative funding, Miller said, there are currently no plans to publish data for 2018.

In its place, we may be left with more unscientific, emotional measures of global terrorism. Even there, however, things are looking calmer: Trump has only tweeted the word “terror” nine times so far in 2018.